Portable chemical analysis systems are required for the detection of explosives and other hazardous material. Such systems may be based on separation by gas chromatography followed by detection, or on ion mobility spectrometry, or on mass spectrometry. Because the analyte concentration is vanishingly low, other devices are often incorporated to improve the detectable limit. One such device is a chemical pre-concentrator, a device for enhancing the concentration of a volatile organic compound (VOC) in a gas stream prior to analysis.
The main components of a pre-concentrator system are shown in FIG. 1. The pre-concentrator element itself is a trap that will preferentially adsorb a dilute VOC from a gas stream. Adsorption is often based on the use of a porous material or a chemically reactive layer of material. Examples of the former are carbon granules and sol-gel glasses, and examples of the latter are functionalized polymers. This material 101 is held on a mechanical support 102, which can be heated. Usually heating is carried out electrically.
The trap is placed in a small enclosure 103 between three valves. The first valve 104 connects to the gas flow input 105, and the second valve 106 connects to the gas flow output 107. The third valve 108 connects to the subsequent analysis system 109. Pre-concentration involves a repetition of adsorption and desorption steps.
FIG. 2 shows the adsorption step. The input and output gas flow valves 201 and 202 are opened, and the valve 203 connecting to the analysis system is closed. A gas stream 204 containing a small fraction of VOC 205 together with a large fraction of other molecules 206 is allowed to pass over or through the trap. Most of the VOC 207 is adsorbed on the trapping layer 208, while the remainder of the gas stream emerges as exhaust 209.
FIG. 3 shows the desorption step. The input and output gas flow valves 301 and 302 are closed, and the connecting valve 303 is opened. The adsorbed molecules are desorbed, usually by rapidly raising the temperature of the chemically sensitive layer 304 using the heater 305, and a concentrated flux of the VOC 306 is passed into the analysis system 307.
Macroscopic pre-concentrators are available commercially. Pre-concentrator performance is defined in terms of the efficiency (i.e., the fraction of the desired analyte that is retained) and of the concentration factor (i.e. the increase in the desired analyte concentration). To maximize the efficiency, the surface area of the trap should be large as possible, and the sensitized coating highly attractive to the desired analyte, while to maximize the concentration factor, dead volumes should be as small as possible.
To reduce cycle times, the heated element should have low thermal mass. However, to increase the concentration factor even further without increasing the time needed for desorption, pre-concentrators are often used in a cascade consisting of a first trap with a large volume followed by a second trap with a small volume. The first trap has high efficiency but a long desorption time while the second trap has a short desorption time. Pre-concentrators containing even more stages are constructed in an analogous way.
The above considerations suggest that pre-concentrators are ideal candidates for miniaturization, and small traps based on capillaries were developed in the 1990s [Mitra and Yun 1993; Feng and Mitra 1998; U.S. Pat. No. 6,112,602]. Increased integration with other components such as valves and gas chromatographs can be achieved by planar processing, and several planar pre-concentrators with thin-film heaters have been developed [U.S. Pat. No. 5,481,110; U.S. Pat. No. 6,171,378]. Micromachined heaters with deep, etched trays filled with sorbent granules have also been demonstrated [Tian et al. 2003; U.S. Pat. No. 6,914,220]. A flow-though pre-concentrator based on a sorbent polymer coating on a perforated heater has also been developed [US 20050095722]. None of these configurations is entirely suitable for a compact system, since the valves needed for overall operation are often added by hybrid integration, causing an increase in dead volume and a reduction in concentration factor.
Accordingly there is a need for an improved pre-concentrator.